1. Field of the Invention
The present invention relates to a transistor and a semiconductor device at least part of which includes the transistor.
In this specification, a semiconductor device means all types of devices which can function by utilizing semiconductor characteristics, and an electro-optical device such as a liquid crystal display device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
Although transistors including silicon semiconductors have been used for conventional display devices typified by liquid crystal televisions, oxide semiconductors have attracted attention as a material which replaces silicon semiconductors. For example, an active matrix display device, in which an amorphous oxide including indium (In), gallium (Ga), and zinc (Zn) is used for an active layer of a transistor and the electron carrier concentration of the amorphous oxide is lower than 1018/cm3, is disclosed (see Patent Document 1).
However, some problems of a transistor including an oxide semiconductor have been pointed out. One of the problems is stability of characteristics, and it is pointed out that electrical characteristics are changed by irradiation with visible light or ultraviolet light.